1. Field of the Invention
The present invention relates to a process for producing a surface emitting laser, a process for producing a surface emitting laser array, and an optical apparatus including the surface emitting laser array produced by the process.
2. Description of the Related Art
A vertical cavity surface emitting laser (hereinafter referred to as VCSEL) has been known as one of surface emitting lasers.
In the surface emitting laser, an active region is sandwiched on both sides thereof by two reflectors to form a resonator in a direction perpendicular to a substrate, and light is emitted in the direction perpendicular to the substrate.
It is important for the surface emitting laser to control transverse mode oscillation. When the surface emitting laser is to be applied to communications, a transverse mode output is required to be a single-mode output.
Therefore, according to the surface emitting laser, a current confinement structure is provided in an inner portion thereof by selective oxidation to limit a light emitting region of an active layer, thereby realizing a single transverse mode.
However, when the single transverse mode is to be realized by only the current confinement structure, it is necessary to reduce the confinement diameter. When the confinement diameter reduces, the light emitting region becomes smaller, and hence it is difficult to obtain larger laser power.
Thus, up to now, there have been studied methods of introducing an intentional loss difference between a fundamental transverse mode and a high-order transverse mode to enable single-transverse mode oscillation while maintaining a light emitting region somewhat wider than in the case where the single transverse mode is realized by only the current confinement structure.
Of the methods, so-called surface relief methods are disclosed in Japanese Patent Application Laid-Open No. 2001-284722 and H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999).
The surface relief methods are methods of performing level difference processing for reflectance control on a surface of a surface emitting laser device, to make a high-order transverse mode loss larger than a fundamental transverse mode loss.
Herein, a level difference structure provided for reflectance control in a light output region of a light emission surface of a reflector as described above is hereinafter referred to as a surface relief structure.
Next, the surface relief structures in the conventional examples described above are described with reference to FIGS. 2A and 2B.
In FIGS. 2A and 2B, reference numeral 200 denotes low-refractive index layers; 202, high-refractive index layers; 204, high-reflectance regions; 206, low-reflectance regions; 208, fundamental transverse mode light distributions; and 210, high-order transverse mode light distributions.
A mirror used for the VCSEL is normally a multilayer reflector in which a low-refractive index layer and a high-refractive index layer are alternately laminated each in an optical thickness equal to ¼ of a laser oscillation wavelength λ so as to form multiple pairs.
In general, the multilayer reflector is terminated at the high-refractive index layer, and hence a high reflectance equal to or larger than 99% is obtained by the use of reflection on a final boundary with air (refractive index=1).
A convex surface relief structure illustrated in FIG. 2A is described. The convex surface relief structure is disclosed in H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999).
As illustrated in FIG. 2A, a part of the high-refractive index layer 202 which is a final layer in the low-reflectance region 206 is removed by a thickness equal to λ/4, and hence the multilayer reflector is terminated at the low-refractive index layer 200. Therefore, the convex surface relief structure is obtained.
According to the convex surface relief structure, a phase of a beam reflected at a boundary between the low-refractive index layer 200 and air which is bottom in refractive index than the low-refractive index layer 200 is shifted by ‘π’ from phases of all reflected beams of the multilayer reflector which are arranged under the low-refractive index layer 200.
As a result, the reflectance in the low-reflectance region 206 is reduced to a value equal to or smaller than 99%, and hence the reflection loss may be made higher than in the high-reflectance region 204.
In order to introduce the loss difference between the fundamental transverse mode and the high-order transverse mode based on this principle, the low-reflectance region 206 is formed in the vicinity of the light output region so that the low-reflectance region 206 largely overlaps with the high-order transverse mode light distribution 210.
In contrast, the fundamental transverse mode light distribution 208 is set so as to largely overlap with the high-reflectance region 204 in which the high-refractive index layer 202 is left as the final layer.
When the surface relief structure is formed as described above, the reflection loss in the high-order transverse mode may be increased, and hence the high-order transverse mode oscillation may be suppressed. As a result, the single-mode oscillation of only the fundamental transverse mode may be obtained.
As illustrated in FIG. 2B, when the low-refractive index layer 200 having a thickness equal to λ/4 is additionally provided on the high-refractive index layer 202 which is the final layer, the low-reflectance region 206 may be formed to obtain a concave surface relief structure. The concave surface relief structure is disclosed in Japanese Patent Application Laid-Open No. 2001-284722.
As described above, even in the case of the concave surface relief structure, the reflectance may be reduced based on the same principle as in the convex surface relief structure, and hence the single-mode oscillation of only the fundamental transverse mode may be obtained.
When the surface relief structure is to be formed, alignment between the surface relief structure and the current confinement structure in an in-plane direction is important.
That is, in order to effectively obtain the single-mode oscillation of the fundamental transverse mode, it is desirable to accurately align the surface relief structure with the current confinement structure which determines a light intensity distribution.
For example, when a central axis of the surface relief structure is shifted from a central axis of the current confinement structure, an unintended loss is introduced to a desired oscillation mode (for example, fundamental transverse mode).
In order to accurately perform the alignment, a method called a self-alignment process is disclosed in H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999).
This method is used to form the surface relief structure and a mesa structure with high precision through alignment patterning using the same mask.
Hereinafter, the self-alignment process disclosed in H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999) is described in detail with reference to FIG. 3.
As illustrated in FIG. 3, a resist is formed on a semiconductor layer 304 and patterned using photolithography to obtain a first resist pattern 300.
An outer region of the first resist pattern 300 is used as a mask for forming the mesa structure, and an inner annular pattern of the first resist pattern 300 is used as a mask for forming the surface relief structure. The shape of the first resist pattern 300 is defined by photolithography, and hence a surface relief structure 302 can be formed with high precision by the inner annular pattern.
When the mesa structure is to be formed by wet etching using the outer region of the first resist pattern 300, the mesa structure with high size precision may be obtained. Specifically, the surface relief structure 302 is formed and then a second resist pattern 306 is formed thereon as a protective layer.
After that, the mesa structure is formed using the outer region of the first resist pattern 300.
The formed mesa structure is oxidized from side surfaces thereof to form the current confinement structure.
As described above, the surface relief structure and the mesa structure can be formed with high precision through alignment patterning using the same mask. As a result, the surface relief structure and the current confinement structure which is defined by the shape of the mesa structure can be also formed with high precision.
According to the conventional production method disclosed in H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999), the central axis of the convex surface relief structure can be aligned with the central axis of a non-oxidized region of the current confinement structure, and hence a device capable of single-transverse mode oscillation can be manufactured.
In the production method disclosed in H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999), the mesa structure (trench structure) is formed by wet etching.
However, in the case where dry etching is required to form the mesa structure, because the resist has a low resistance to dry etching, a problem occurs in processing precision when the mesa structure having a certain level of height is to be formed.
In particular, in the case of a short-wavelength VCSEL (for example, 680 nm band), the number of pairs in the multilayer reflector serving as a top reflector is large. Therefore, the height of the mesa structure to be formed becomes high, and hence dry etching is used instead of wet etching. Thus, the method disclosed in H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999) has a problem in terms of processing precision.